This invention relates to thermal ink jet printing, and more particularly to heater elements for thermal ink jet printheads.
Ink jet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer. An ink jet printer typically includes a printhead having a multitude of independently addressable firing units. Each firing unit includes an ink chamber connected to a common ink source, and an ink outlet nozzle or orifice. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles.
In thermal ink jet pens, the transducer is a resistive heater element that provides sufficient heat to rapidly vaporize a small portion of ink within the chamber, forming a bubble. The bubble displaces a droplet of liquid ink from the nozzle. For uniform and precise printer output, it is desirable that the timing, magnitude, rate, shape, and position of the bubble formation be as uniform as possible. Uniformity is desired from firing unit to firing unit, and between sequential droplets originating from the same nozzle.
A particular uniformity concern relates to the boiling properties of fluids. Heterogeneous nucleate boiling, or bubble nucleation, normally occurs at defect sites on the surface of a heating element, or other heated surface. These defects may be cracks, discontinuities, and edges and vertexes where surfaces meet at angle. Heterogeneous nucleate boiling occurs more readily than homogenous or film boiling, which occurs after additional heat energy is added when sufficiently sized nucleation sites are not present. Therefore, it is the heterogeneous nucleation that has the greatest effect during the rapid and uniformity-sensitive boiling process that occurs during thermal ink jet printing.
Existing thermal ink jet printheads have at least partially controlled the heterogeneous nucleate boiling process by providing each firing chamber with a heating element shaped with a single small recessed basin having sharp edges that provide nucleation sites. The basin is smaller than the respective orifice, and registered therewith so that all potential nucleation sites are directly below an open portion of the orifice. This avoids the risk that some firing chambers may lack any nucleation sites and require a higher energy to achieve homogeneous nucleation. The deliberate positioning of the sites in registration with the orifices also reduces the chance that an unintended defect offset from the centerline will generate off axis droplet ejection. However, these improved systems have not achieved ideal uniformity of performance.
The uniformity disadvantages of prior art systems are reduced or overcome by providing a thermal ink jet with a body having an ink firing chamber and an orifice. An electrically activated heating element is connected to the body in thermal communication with the firing chamber, and includes a contoured surface portion coextensive with at least a portion of the heating element. The contoured surface portion of the firing chamber has a plurality of recesses.